In Godzilla: Generations, there are basically two games. One is this one, that everyone knows about. The other is... well, this:
After the opponent is defeated, your character seems to transform into another, which I guess is how you unlock other kaiju. Note that you don't transform into the kaiju you defeated, but one that appears to be chosen at random (for instance, defeating King Ghidorah as Godzilla unlocks Ebirah, at least in the battle I watched).
There are an impressive 44 different characters in this game (35 classic monsters (plus an egg [???]) and 8 new characters), making this game one of the largest Godzilla games as far as the cast goes. I know nothing on the original kaiju (such as if they are even original kaiju. Maybe they're from something else, but all I can tell you is that whatever it was isn't related to Godzilla. Never seen them before). I had to translate those names myself. Don't know what the deal is with "Metal Godzilla", either.
There's a shocking lack of information on this. Not having the game myself, I'd like to know more. If anyone has info on those new kaiju, please post it.